At a first glance, the experimental set-up of Marpaung and his team looks like many others in the room: a black curtain separates the lab table full of thin optical fibers, a microscope and complicated-looking equipment. But the scientists perform unique experiments here, that may change the way how digital information is processed. It’s all about a tiny chip, about 1.5 by one centimeters, that is placed under a microscope. The nearby computer screen shows the chip in full frame. Tiny lines that are edged into the surface are clearly visible and form a pattern resembling a race track.
‘In principle we use infrared laser light to test and further develop this chip, but to make the light path visible, we use red light,’ Marpaung explains. He switches on the light source, and invisible to the eye, red light rushes through the thin optical fibers, and passes on to the attached chip. Marpaung points to the screen where the race track light up in red: ‘You see how the light is guided through these small lines, these are the special light channels that are the basis of our new chip design.’
Highways of information
Integrated circuits (ICs), or chips, are a crucial part of modern life. Their low price allowed mass manufacturing, and has revolutionized electronic equipment. A chip consists of many electronic circuits, that are edged on a flat piece of silicon. They function as highways for information transfer and are indispensable for modern technology, from computers, mobile phones, cars and many home appliances to space crafts and satellites. Over time, chips have become smaller and smaller, while their performance increased, allowing for better and faster electronics.
The next big thing in IC technology is the so-called photonic chip. These IC’s transport information through light, not electricity, through optical channels, or optical waveguides, that can be seen as ‘highways for light’. This light-based information transfer has several advantages over traditional ICs and has a huge potential to improve current electricity-based ICs.
'These advantages are particularly important for large data centers'
‘Light-based ICs are smaller and pack and process more information, while consuming much less energy than traditional ICs,’ Marpaung says. ‘These advantages are particularly important for large data centers, where limitations in data transfer and huge energy costs are increasing problems.’ An additional advantage is their smooth integration in existing electronic devices, since their basic way of performing are similar to traditional ICs.
An improvement of the current photonic chips is however possible. Scientists found out that when acoustic waves are combined with light waves, their interaction might result in even better information management and transfer than in photonic chips. ‘When you introduce a sound wave in a photonic IC, which is essentially a mechanical wave travelling a lot slower than a light wave, a whole new array of possibilities to manage the information arises,’ Marpaung says. ‘The slower speed of the sound wave results in a much higher accuracy and resolution to process information. It allows you to easier filtering out noise, amplify the signal and select information or data you want to keep. This could result in a cleaner signal with less impact of noise.’
Manipulate light waves
However, to create a sound wave inside a minuscule IC is a challenge by itself. The scientists managed to do this by using two laser beams, with different frequencies. By aiming the laser beams inside the optical waveguide, they influence each other: at some points, the laser beams weaken each other, while at other points they strengthen each other: a process called interference.
‘At the places where the lasers enhance each other, they create a force, that acts like a hammer,’ Marpaung explains. ‘This force briefly compresses the material of the light channel and creates some kind of ‘bang’: a sound wave is born.’ Together with the light, the sound wave travels through the light waveguide, compressing the surrounding light channel material. This compression also influences and changes the speed of the light travelling through. So, by modulating the sound, the scientists can also manipulate the light waves travelling through the chip, and thus process the light-based information transfer.
While the theory holds, there are some problems in practice. It is a great challenge to build a chip containing light waveguides, that also can guide sound waves well. This is needed to have a proper interaction between the light and the sound. For example, in a modern silicon photonic chip, the channels functioned well for light, but not for sound waves. Other chip materials may show the opposite. So, the scientists need to build a light channel that is suited for both light and sound waves. A very complicated task. They solved the issue by slightly manipulating the design of standard photonic chips. The optical waveguides in these standard ICs are composed of two different materials, optimized to guide the light: a solid core of silicon nitride, where light travels slowly, surrounded by a glass mantle, where light travels fast.
Marpaung: ‘In the end, the light is guided through and kept within the core, with lower speed, because it bounces between the material interfaces due to total internal reflections. If there was only one material, the light would scatter everywhere, and there would be no guiding.’ However, for sound waves the opposite is true: they travel faster inside the core, resulting in massive escaping from sound waves outside the core. This prevents an interaction between light and sound waves, so light and sound waves cannot be combined effectively in the traditional wave guide design.
Stable and mature design
Professor Marpaung and his team came up with a brilliant, alternative design of the standard optical wave guide that solved the issue. ‘We made a waveguide containing not one, but two cores of silicon nitride inside the glass mantle,’ he explains. ‘When the light travels through both these cores, there is some light leakage in between the cores into the glass mantle. If we create the sound wave also between the cores, inside the glass that guides it well, we keep the sound and light waves in one volume, allowing the sound to interfere with the light.’
'This new design is basically the proof of principle'
According to the scientist, the beauty of the solution is that it is based on a stable and mature design optical wave guide, that can easily be modified for more applications. ‘This new design is basically the proof of principle,’ says Marpaung. ‘After modifying and scaling our basic design, there will be many applications, because the sound wave can filter and select information in high resolution and precision. It can for example be used in space and defense technology, next generation communication technology, and, not to forget, quantum computers. Because of the promising applications, our team has received 2.5-million-euro EU money through a very prestigious ERC Consolidator grant!’